[PATCH 2/2][Aarch64] Improve FP to int conversions

[Michael Collison]

This patch improves additional cases of FP to integer conversions with -ffast-math enabled.

Example 1:

double
f5 (int x)
{
  return (double)(float) x;
}


At -O2 with -ffast-math

Trunk generates:

f5:
	scvtf	s0, w0
	fcvt	d0, s0
	ret


With the patch we can merge the conversion to float and float-extend and reduce the sequence to one instruction at -O2 and -ffast-math

f5:
	scvtf	d0, w0
	ret

Example 2

int
f6 (double x)
{
  return (int)(float) x;
}


At -O2 (even with -ffast-math) trunk generates

f6:
	fcvt	s0, d0
	fcvtzs	w0, s0
	ret

We can merge the float_truncate into the fix at the rtl level

With -ffast-math enabled and -O2 we can now generate:

f6:
	fcvtzs	w0, d0
	ret
	
Bootstrapped and regression tested on aarch64-linux-gnu. Okay for trunk?

2018-05-15  Michael Collison  <michael.collison@arm.com>

	* config/aarch64/aarch64.md:
	(*<optab><mode>df2): New pattern.
	(truncdfsf_<optab><mode>2: New pattern.
	(*fix_to_sign_extend<mode>di2): Ditto.
	* gcc.target/aarch64/float_int_conv.c: New testcase.
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